The present invention relates to axial flow turbines and in particular to aerodynamic design aspects of such turbines for improved efficiency. The invention relates, more specifically, to airfoil, nozzle and exhaust duct shape and configuration.
Axial flow gas turbine engines, for example, normally comprise a compressor, a combustion section, and a turbine section. To these are added intake and exhaust systems. Atmospheric air is drawn into the compressor through the intake system and is then passed at high pressure into the combustion section, where it is mixed with fuel and the mixture ignited to create a working fluid in the form of a pressurized hot gas. This passes to the turbine section where its energy is converted by the turbine blades into useful work. The temperature and pressure of the working gas is now considerably diminished, and is discharged to atmosphere by the exhaust system.
The turbine section consists of rotor blades and stator blades. To distinguish between the two, unless the context otherwise dictates, the rotor blades will be referred to as xe2x80x98bladesxe2x80x99 and the stator blades will be referred to as xe2x80x98vanesxe2x80x99. The blades and vanes form a series of axially successive annular rows. Each blade is attached to a turbine rotor disc or drum via a portion known as the root. The disc or drum is mounted on a rotor shaft whose centre line defines the rotational axis of the turbine. The vanes are fixed, typically to an inner drum and/or an outer turbine casing, such that they alternate with the rotor blades to form paired rows of vanes/blades. Each such pair of rows forms what is known as a turbine stage, in which the vane is followed in axial flow succession by the blade. A turbine may comprise one or more stages, and it is common for the turbine to comprise high pressure and low pressure sections, each section containing one or more stages.
The blade rows extract energy from the working fluid and transfer it to the turbine rotor, whereas the vanes smooth the passage of the working fluid and direct it at an optimum outlet angle so as to meet the rotating blades at the designed angle. In this way energy transfer is carried out as efficiently as possible.
Vanes and blades of axial flow turbines have a cross-section profile of the generic airfoil type and bear a strong visual likeness one to another, notwithstanding scale differences usually dependent upon engine size. However, on inspection it is found there are measurable differences of airfoil profiles, not only between engines of different make and type, but also between turbine stages of the same engine. Further, such differences may have significant effect on turbine efficiency. Similarly, there are differences in other aspects of turbine stage design which alone or in combination also have an effect. Small differences in such design features, which may appear minimal or unimportant to those unskilled in the art, may in fact have a significant effect on turbine stage performance. Turbines currently operate at very high efficiency values, the best of which are in the region of 90%. At this level of efficiency, it is very difficult to make improvements, so even improvements as small as 1 or 2% are regarded as significant.
In part, the present invention incorporates and improves upon previous teachings in respect of so called xe2x80x9cControlled Flowxe2x80x9d principles of airfoil design by the present inventor and others. In particular, see United Kingdom Patent No. GB 2 295 860 B, and United Kingdom Patent No. GB 2 359 341 B. These two prior patents should be read to gain a full appreciation of the present invention. Other patents showing similar principles include U.S. Pat. No. 5,326,221 to Amyot, et al., (for steam turbines) and U.S. Pat. No. 4,741,667 to Price, et al., (for gas turbines).
In an improved turbine according to the present invention, efficiencies as high as 93% may now be possible. The invention also facilitates a design that is more compact, thereby reducing its footprint size and making potentially large savings in terms of space and therefore cost.
To aid understanding it will be useful to include at this stage some definitions of terms or expressions utilized in the following description.
In axial flow turbines, the xe2x80x9crootxe2x80x9d of a blade is that radially inner part which is attached to the rotor structure, whereas the radially outer opposite portion is the xe2x80x9ctipxe2x80x9d. For the purpose of describing the present invention, the radially inner ends of the airfoils of both the vanes and the blades will be called the root sections and the radially outer ends of their airfoils will be called the tip sections.
Airfoil cross-sectional profiles will be defined by reference to an x,y co-ordinate system as illustrated in FIG. 7 of the accompanying drawings, where xe2x80x98xxe2x80x99 is the axial co-ordinate as measured along the rotational axis of the turbine and xe2x80x98yxe2x80x99 is the tangential co-ordinate as measured along the instantaneous direction of motion of the rotor blade. To reduce the bulk of the Tables incorporated herein, airfoil cross-sectional profiles will be numerically defined in x,y co-ordinates at only three radial stations along the radial co-ordinate xe2x80x98zxe2x80x99, these being profiles at the airfoil root end, mid-height and tip end. However, profiles between these stations can be readily obtained by those skilled in the art by a process of interpolation and smoothing.
The expression AN2 represents the product of the area A of the annulus swept by the LP turbine blade airfoils at the outlet of the stage, multiplied by the square of the rotational speed N of the blades. The annulus area itself is defined as the difference in area of the circles delineated by the inner and outer radii of the blade airfoils. Exemplary numerical values for typical prior art turbines and a turbine produced according to the present invention will be given in the appropriate section of the following description.
The pitch dimension of a row of blades or vanes is the circumferential distance from one airfoil trailing edge to the adjacent airfoil trailing edge in the same row at a specified radial distance from the root end of the airfoil.
The axial width (W) of an airfoil is the axial distance between its leading and trailing edges, as measured along the rotational axis of the turbine. The pitch/width ratio (P/W) at the root perimeter is an important parameter which influences the efficiency of the blade or vane row, the number of blades or vanes (and therefore the cost) and the circumferential width of the rotor disc-post (i.e., it affects rotor disc stressing). In this connection, note that gas turbines typically operate at very high rotational speeds (for example 17,400 rpm). This can generate very high centrifugal forces, reaching 110,000G at the rotor tips. Both blades and vanes are also subject to very high temperatures. Turbines must be designed to withstand the stresses imposed by these conditions of use.
The tip/hub diameter ratio is an indicator of the comparative radial length of the blades compared to the overall diameter of the turbine. Its significance is that it represents the annular area available for passage of the working fluid.
Turbomachinery efficiencies compare the actual changes in the fluid between inlet and exit with the theoretical best xe2x80x98reversiblexe2x80x99 change. Also, at the exit from the last stage, either total or static conditions can be used. This gives either xe2x80x98Total to Totalxe2x80x99 or xe2x80x98Total to Staticxe2x80x99 efficiency. The difference is the exit kinetic energy of the gas.
In concert with new airfoil designs, a turbine according to the invention includes an improved turbine nozzle shape.
Consider a gas turbine having a single high pressure (HP) stage followed by a low pressure (LP) section, the low pressure section including a plurality of individual stages. Known types of last LP stage discharging into an exhaust system tend to generate a non-uniform leaving energy and stagnation pressure profile which is detrimental to the overall performance of the last stage and exhaust. Hence, it would be advantageous if the last LP stage could generate a stagnation pressure profile into the exhaust which is nearer the ideal, this profile being virtually constant across the span and increasing slightly towards the tip.
The exhaust of a gas turbine is the final stage of the flow path expansion and is responsible for efficiently discharging the spent working fluid from the turbine into the atmosphere. Current turbine exhaust designs achieve 60% pressure recovery with an exhaust having a length (L) to last LP blade height (H) ratio (L/H) approximating to a figure between nine and ten. In this context xe2x80x98lengthxe2x80x99 represents the axial length of the exhaust from the final low-pressure stage to the downstream end wall of the turbine, whereas xe2x80x98heightxe2x80x99 represents the radial height of the last blade airfoil in the low-pressure stage of the turbine. As previously mentioned, the footprint of a turbine is a measure of the cost of its installation. For example, current cost for a turbine installation on an oilrig (say) is £80,000 (British Pounds) per square meter. Hence, it is desirable if a compact design can be achieved.
The design and constructional features of the various aspects of the invention and their advantages over prior turbine designs will now be explained with reference to the following sections of the specification.
The invention comprises, in a first aspect, an gas axial flow gas turbine comprising in axial succession a turbine and a turbine exhaust section, the turbine comprising a turbine nozzle containing a low pressure turbine stage having an annular row of stator vanes followed in axial succession by an annular row of rotor blades, wherein the low pressure turbine stage is characterized by the following parameters:
the ratio of vane airfoil pitch to vane airfoil axial width at the root end of the vane airfoil (P/W) is in the region of 1.0 to 1.2;
the ratio of blade airfoil pitch to blade airfoil axial width at the root end of the blade airfoil (P/W) is in the region of 0.6;
the ratio of blade diameter at the tip end of the blade airfoil to blade diameter at the root end of the blade airfoil is in the region of 1.6-1.8; and
the ratio of the axial length of the exhaust section to the blade airfoil height (L/H) is no greater than a value in the region of 4:1;
said parameters being subject to a predetermined amount of variation.
Preferably, the above ratio of vane airfoil pitch to vane airfoil axial width at the root end of the vane airfoil (P/W) is about 1.12 and the above ratio of blade diameter at the tip end of the blade airfoil to blade diameter at the root end of the blade airfoil is about 1.72. The ratio of the axial length of the exhaust section to the blade airfoil height (L/H) is preferably about 3:1. The predetermined level of variation may be up to xc2x110%, preferably xc2x15%.
The turbine stage vane and blade airfoil cross-sectional profiles at the root, mid-height and tip may be as defined according to Tables 1A to 1C and Tables 2A to 2C respectively, subject to said predetermined level of variation. The values listed in these tables may be scaled by the application of suitable scaling factors (as known per se) to obtain turbines able to deliver larger or smaller powers.
The configuration of the turbine nozzle may be as defined herein by reference to Table 3 and FIG. 3; and the exhaust section configuration may be as defined herein by reference to Table 4 and FIG. 6. To match airfoil configurations which have been scaled from Tables 1A to 2C, Tables 3 and 4 may be utilized to define the respective shapes of the turbine nozzle and exhaust section, their actual dimensions being adjusted to be commensurate with the scaled x, y and z values of the vane and blade airfoils, as appropriate.
The blade airfoils may be hollow, to reduce weight. Preferably, they taper so as to have a smaller axial width at their tips than at their root ends. However, the vane airfoils should preferably taper in the opposite direction so as to have a larger axial width at their tips than at their root ends.